In a liquid-effluent-free pulp mill, bleach plant effluent, containing sodium chloride values from chlorine-containing spent bleaching chemicals, typically chlorine, chlorine dioxide, chlorine monoxide and sodium hypochlorite, and soda values from spent caustic extraction chemicals, typically sodium hydroxide, used in the bleaching and purification procedures of the bleach plant, is discharged to the spent pulping liquor recovery and regeneration operation of the pulping process producing the pulp for bleaching and purification.
Typically such recovery and regeneration operation includes burning the liquors to remove carbonaceous materials and to form a mixture of chemicals reusable as or convertible into pulping chemicals for recycle. Since sodium chloride is unaffected chemically by the recovery and regeneration operation, it would remain as a dead load in the effluent-free pulp mill and build up in the system unless steps are taken to remove the same.
The effluent-free pulp mill may be used with the Kraft mill process, wherein the active pulping chemicals are sodium hydroxide and sodium sulphide. Sodium sulphate or other sodium-and sulphur-chemical is added to the spent pulping liquor to make up losses of sodium and sulphur from the system. The recovery and regeneration operation produces a smelt in the burning or furnacing step which contains mainly sodium sulphide and sodium carbonate which later is converted to sodium hydroxide, together with small amounts of sodium sulphate and other sodium sulphur-oxygen compounds. The smelt also contains sodium chloride in the effluent-free mill procedure.
The present invention is directed to the removal of sodium chloride in pure form from the smelt without removing the usable chemicals. Canadian Pat. No. 928,008 describes a procedure for the removal of sodium chloride from a sodium sulphide-, sodium carbonate- and sodium chloride-containing smelt involving first leaching sodium sulphide from the smelt at a temperature above 50.degree.C followed by separation of sodium chloride from the sodium sulphide-free smelt at a temperature below 20.degree.C.
Problems exist with this procedure. Thus, under the conditions of operation, the sodium chloride is obtained as an aqueous solution thereof which also contains some dissolved sodium carbonate. This prior art suggests discarding the sodium chloride solution, thereby discarding some of the valuable sodium carbonate.
The present invention seeks to overcome this problem of the art by recovering the sodium chloride in a solid substantially pure form and avoiding the discarding of sodium carbonate values from the system.
The principles of this invention are applicable to other pulp mill operations in addition to the Kraft mill process which utilize sodium sulphide or in which aqueous sodium sulphide is produced as an intermediate product. Typical of such additional processes are high yield pretreatment Kraft, polysulphide, alkafide and sodium-based sulphite processes.
The effluent-free pulp mill also may be used in a soda mill where sodium hydroxide is used as the active pulping chemical. The furnacing in the recovery and regeneration operation results in a smelt which contains sodium carbonate and sodium chloride.
The smelt also usually contains small quantities of sodium sulphate and other sodium- and sulphur-oxygen salts arising from the furnace fuel oil, the wood and from sodium sulphate-contaminated make-up chemicals.
The present invention is also directed to the recovery of sodium chloride in substantially pure solid form without loss of usable chemicals from the sodium carbonate-, sodium chloride- and sodium sulphate-containing smelt of a soda mill.
While the present invention will be described more particularly with reference to the presence of sodium chloride in the smelt arising from bleach plant effluents, the sodium chloride may arise from other sources, singly or in combination, such as sodium chloride in the wood chips from which the pulp is formed, arising from the floating of logs or sea water, sodium chloride contamination in pulp mill chemicals and sodium chloride contamination in water utilized in the pulp mill.